The standard interpretation of electocardiographic data does not allow differentiation of certain types of rhythm disorders, necessitating invasive testing. We believe that there is additional information contained in the ECG, in the form of small but systematic phase and amplitude changes that may provide important localization information. We will use inversion theory and modern signal processing methods to address a set of clinically interesting problems involving early excitation and re-entrant events. In Phase 1 as proof of concept, we will study atrial tachycardias originating at the superior vena cava versus right pulmonary vein in order to provide information on the source of the abnormal rhythm. We make this choice for our initial study in part because of its clinical significance, in part due to the relative isolation of the P wave from other parts of the ECG, and in part due to the expected success in differentiating candidate paths. We will obtain digitally recorded 12- lead ECG data from subjects during ablation procedures. We anticipate that 1/2 of the data will be used as a training set and 1/2 reserved as a test set. Our principal Phase 1 goal is to develop prototype code to differentiate arrhythmia sources and to test the method on clinical data. The method will be considered successful if it shows a better success rate than conventional methods on the same data, or if it shows a comparable success rate with clinically significant improvement in resolution compared to conventional methods. The principal commercial application of this research is the production of a modular, commercial quality software algorithm suitable for inclusion in an ECG analysis system. The initial focus will be on atrial tachycardias originating at the superior vena cava versus the right upper pulmonary vein. If this is successful, we will expand our methods to include differentiation between candidate sites for a wide variety of atrial and ventricular arrhythmias. We also note that if the method is successful, it may be possible to extend the results to more complicated arrhythmias and re-entrant events involving multiple entry points, or events obscured by other, larger phases.